There are presently in existence in the United States approximately 190,000 emergency locating transmitters (ELTs) required by law on all small aircraft travelling more than 25 miles from an airport and about 6,000 emergency position indicating radio beacons (EPIRBs) required on certain classes of marine vessels. ELTs and EPIRBs are essentially the same device which transmits an audio tone on legislatively assigned frequencies of 121.5 MHz and 243 MHz indicating that a distress incident has occurred. The audio tone generated by these devices is provided by a distress modulation signal legislatively assigned to have a 2 Hz to 4 Hz cyclic waveform wherein each cycle has a downward sweep of at least 700 Hz between 1,600 Hz and 300 Hz. The distress waveform demodulated in a conventional AM receiver provides a sirenlike audio tone that is recognized by distress band observers. The Air Force Rescue Coordination Center (AFRCC), for example, receives information from others, i.e., FAA flight service stations, airports, the Civil Air Patrol, etc., who monitor the 121.5 MHz and 243 MHz frequencies and respond to distress signals by notifying search and rescue personnel, (SAR), such as the Civil Air Patrol, to search for the location of the distress transmission and initiate rescue operations. Since the distress transmission contains no information other than the fact of a distress, however, the SAR have no way of determining the time of occurrence of the distress. Knowledge of the incident time of the distress would, for example, enable the rescue coordinator to assign priorities and resources in a multiple emergency situation so that the emergencies that are critical from a time survival relationship are attended to early. Knowledge of the duration of the emergency incident also enables correlation of incident time to proximity weather phenomena or prevailing tides. Statistics can be compiled based upon these parameters and rescue times to develop predictive data.
Satellite aided search and rescue systems are presently being developed to augment existing search and rescue force capabilities to detect and locate ELT/EPIRB signal sources by improving the distress monitoring coverage of the Continental United States, Alaska, the U.S. maritime areas, Canada and the Canadian maritime areas, and by improving the position location determination accuracy of the distress incident using Doppler techniques. The orbiting satellites will respond to low level 121.5 MHZ distress signals as well as high level 406 MHz data signals in a form specified in the U.S. by the Federal Communications Commission and internationally by the Radio Regulations of the International Telecommunication Union. The 406 MHz information bursts contain information concerning the distress incident, such as user identification, country of origin and situation, e.g., ship sinking. The 406 MHz information is either processed on board in the satellite or relayed to ground based instrumentation for processing. The low level 121.5 MHz signal is generated during the off periods of the 406 MHz information bursts to enable SAR to perform a final location following course location determination by satellite to within a radius of about one kilometer.
One object of the present invention, therefore, is to provide a method of and system for encoding an ELT/EPIRB transmission with distress incident time information.
Another object of the present invention is to provide a method of and system for identifying the distress incident time in an ELT/EPIRB transmission.
Another object of the present invention is to provide a method of and apparatus for encoding an ELT/EPIRB transmission with data identifying the elapsed time of the distress.
Yet another object is to provide a method of and system for encoding information burst signals generated by an ELT/EPIRB to include distress incident time information.
Still another object of the invention is to provide a method of and system for providing distress incident time information in an ELT/EPIRB transmission that is compatible with present ground based as well as satellite augmented search and rescue operations.
The above and still further objects, features and advantages of the present invention will become apparent upon consideration of the following detailed description of specific embodiments thereof, especially when taken in conjunction with the accompanying drawings.